The field of the disclosure relates generally to actuators and motors and, more particularly, to linear switched capacitance actuators and motors.
Many known motors/actuator devices use magnetic fields as a force transfer mechanism rather than electric fields due to the higher energy densities achieved with magnetic fields using conventional materials and configurations. Such known devices are sometimes referred to as electromechanical actuators (EMAs). At least some of these EMAs include at least one electric motor as a driving device, such motor coupled to one of an alternating current (AC) power source and/or a direct current (DC) power source. Some of these known motor-driven EMAs may also include a power transfer device, e.g., a geared transmission or a direct drive shaft. The motor may be powered through power electronics, e.g., insulated-gate bipolar transistors (IGBTs) to facilitate increases in operational efficiency or implement complex control tasks. Many other known EMAs are hydraulically-driven and include an accumulator and a hydraulic pump/motor combination. Such known EMAs are used extensively for operation of larger devices such as valves and dampers. However, they have some disadvantages for smaller applications, such as operation of robot translatables and aviation devices.
At least some other known motors and actuators use electric fields rather than magnetic fields for electro-mechanical energy transfer. A switched capacitance actuator (SCA) is an electric field-based device that demonstrates an improved energy density over earlier electric field-based devices. The electro-mechanical energy conversion is at least partially a result of the change in the device capacitance with respect to rotor translation. Such SCAs are electrostatic motors that include a translatable portion, e.g., a rotor and a stationary portion, e.g., a stator and operate in a manner similar to the magnetic field equivalent of the SCA, a switched reluctance motor (SRM). Both the rotor and stator include multiple electrodes that correspond to magnetic poles in a SRM. When voltage is applied to a stator capacitor electrode pair, a rotor electrode will induce rotation in the rotor to align with the stator capacitor electrode pair. When the voltage on this stator electrode pair is removed, the appropriate next stator electrode pair that is not aligned with the rotor electrode is energized with a voltage to continue the rotational motion. Thus an external switching circuit is required to switch the stator excitation, though the machine may be configured to operate synchronously with three-phase sinusoidal excitation.
Such SCAs offer advantages over magnetic EMAs in that continuous electric current is not required to generate torque, thereby decreasing overall power consumption. Also, many standard components of magnetic EMAs, e.g., an iron core-type as a magnetic conductor and a yoke (or equivalent) are not required. Also, such SCAs require much less copper conductor. As such, the size, weight, efficiency, and cost of SCAs may be much lower than those for magnetic EMAs. The improved efficiency is also partially due to the decrease in losses of the SCAs which include thermal, mechanical, and electromagnetic losses. Since the copper losses in the SCA are smaller than in conventional machines and the dielectric losses can be held small compared to iron losses, the efficiency of SCAs is improved.
However, such known SCAs do not match electromagnetic machines with respect to the motion inducing shear stress, i.e., total force or torque output per unit rotor surface area. Typically, magnetically coupled actuators have gravimetric power densities below 1 kiloWatt per kilogram (kW/kg). In comparison, typical hydraulic actuators have gravimetric power densities on the order of 3-5 kW/kg, however, such typical hydraulic actuators have low efficiencies. Therefore, to attempt to achieve parity with electromagnetic devices with respect to power-to-weight ratio, at least some known SCAs compensate for the relatively lower shear stress by increasing the active area of the air gap defined by the SCA rotor and stator. According to Gauss' divergence theorem, electric field lines are not required to define closed field loops, in contrast, magnetic field lines form closed loops that originate and terminate on the magnet. Since the electric field lines do not need to be closed, the rotor surface area may be increased by adding active layers. Another strategy to increase the power-to-weight ratio is to increase the shear stress by improving the dielectric breakdown strength within the gap of the SCA. For example this may be achieved through evacuating the SCA casing. The dielectric breakdown strength of vacuum is much higher than that of air, which facilitates the strength of the electric fields in the gap to be larger. However, the evacuation configuration increases the complication of the SCA since the device needs to be securely sealed with a vacuum pump. Such a configuration is difficult to implement in robotic and aviation applications, at least partially due to size and weight constraints.